Fatigue damage of various materials has been a topic of considerable economic importance and one to which a significant amount of scientific and engineering effort has been devoted over the latter half of this century. Methods for non-destructive testing are of particular interest.
Fatigue damage is intimately tied to microscopic structural and chemical processes that occur at the points of microdeformations, which develop over time as a material is strained. These processes have been used advantageously as the basis of analytical techniques that measure fatigue damage within metallic materials under simulated service conditions. For example, under conditions of applied cyclic deformation, chemical processes such as the oxidation, leading to passive film formation, and reoxidation, subsequent to passive film rupture, of a metal substrate at points of microdeformation can be probed electrochemically.
Electrochemical fatigue measurement is the subject of U.S. Pat. No. 5,419,201, entitled "Methods and Devices for Electrochemically Determining Metal Fatigue Status", to Li and Laird. This measurement technique uses a three-electrode electrolyte-filled cell to provide an electrochemical interface with the material under test. The material is subjected to cyclic mechanical deformations, and during these deformations, the transient current passing through the electrolyte are measured. The current's signature is analyzed to determine the fatigue status of the material.
Another electrochemical technique that has recently been studied is that of electrochemical impedance spectroscopy. "Impedance", in its most general sense, can be the quotient of any cyclic input perturbation (e.g., mechanical, electrical or optical) and a consequent cyclic output response. A "true" electrical impedance is measured by applying an ac voltage to the material, measuring the response current and phase relationship, and calculating the impedance (Z=V/I). An electrolyte-filled cell having two or three electrodes is used to apply the input voltage and to measure the output current. Impedance values over a suitable spectrum of frequencies can be related to physical and chemical properties of the material.